PGAdmin is a WEB- GUI for Postgres Databases e.g. for Nextcloud.

To Setup that Service, create a Folder named pgadmin in your docker_compose directory you created before when setting up docker

Create the docker-compose.yml like this:

services:
  pgadmin:
    image: dpage/pgadmin4
    environment:
      PGADMIN_DEFAULT_EMAIL: admin@domain.tld
      PGADMIN_DEFAULT_PASSWORD: NEWPASSWORD
    volumes:
      - pgadmin_data:/var/lib/pgadmin/
    networks:
      - nextcloud-aio
      - default

volumes:
  pgadmin_data:
    driver: local

networks:
  nextcloud-aio:
    external: true
  default:
    driver: bridge
    enable_ipv6: true

Append to the Caddyfile of docker-caddy

# pgadmin - Postgres- Administration
https://pgadmin.domain.tld:443 {
        header Strict-Transport-Security max-age=31536000;
        reverse_proxy servername-pgadmin-1:80
}

Thats all. Restart Caddy Container and Navigate to pgadmin - you need to configure your Server with internal Docker Servicenames e.g Server Hostname nextcloud-aio-database. For Nextcloud, the Database- Password will be in the Volume Nextcloud in the config.php of nextcloud.